<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .a1{ background: green; width: 200px; height: 200px;
        float: left}
        .a2{ background: red; width: 200px; height: 200px;
        float: left}
        .a3{ background: blue; width: 200px; height: 200px;
        float: left}
        .a4{background: yellow;width: 200px;
            height: 200px;
            clear: both;/*清除浮动*/
            float: left;}
    </style>
</head>
<body>
<div class="a1"></div>
<div class="a2"></div>
<div class="a3"></div>
<div class="a4"></div>

</body>
</html>